1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and particularly to a method of manufacturing a semiconductor device wherein stable and high heat-resistant silicide is formed with high productivity by using a salicide process.
2. Description of the Related Art
A silicide structure using high melting-point metal has been hitherto proposed in order to reduce an electrical resistance of impurity diffusion layers and polysilicon layers of semiconductor devices, and particularly a salicide forming process using high melting-point metal such as Ti (titanium) has been used. According to the salicide forming process, an element isolation oxide film, a gate oxide film and a gate polysilicon electrode are formed on a silicon substrate, then a side wall made of an insulating film is formed on the lateral surface of the gate polysilicon electrode, and then source/drain diffusion layers are formed. In such a state that these diffusion layers and the polysilicon gate electrode are exposed to the outside, As (arsenic) is doped to make amorphous the surface area of the diffusion layers and the gate polysilicon electrode, and then a pure Ti film is formed at a temperature of 300.degree. C. or more by a sputtering process and then subjected to a ramp anneal heat treatment to form Ti silicide (TiSi.sub.2) having C49 crystal structure on the surfaces of the diffusion layers and the polysilicon gate electrode. Thereafter, those portions which are not silicified on the oxide film are selectively removed by a wet etching treatment, and further the ramp anneal treatment is performed again to make TiSi.sub.2 have stable C54 crystal structure having reduced resistance.
The recent microstructure (minute) design of devices has caused the junction of the diffusion layer to be shallow, and when the junction of the diffusion layer is shallow, it is required that the thickness of silicide on the diffusion layer is set to 50 nm or less. In addition, the dimension in the lateral direction is reduced, so that the width of the diffusion layer is reduced and also the gate width is reduced. Therefore, TiSi.sub.2 agglomerates on the diffusion layers and the gate because the surface energy of TiSi.sub.2 becomes smaller when it agglomerates rather than it forms a film on silicon. Therefore, the silicide portion and the Si portion are arranged in the spot form, and thus the resistance is increased and the dispersion of the resistance is also increased. In addition, since thin film silicide is more liable to agglomerate as the temperature increases, it starts agglomeration when the heat treatment is performed at 750.degree. C. or more, and the resistance increases more greatly and the dispersion thereof becomes more remarkable particularly in the case of narrow line patterns. In the present CMOS process, there are some cases where the process using a temperature of 750.degree. C. or more is needed after silicide is formed. Further, even in a case where the silicide process is applied to another device, the application of silicide is greatly limited because the silicide has no resistance to heat of 750.degree. C. or more.
As described above, TiSi.sub.2 has the lowest specific resistance among suicides, however, the heat resistance thereof is lowered. Accordingly, in order to enhance the heat resistance with suppressing the increase of the resistance value as much as possible, it is expected that the resistance could be suppressed and the heat resistance could be enhanced if an alloy of Ti and metal of high melting point is used in place of pure Ti. From this viewpoint, an alloy of Ti and W (tungsten) or Ta (tantalum) has been proposed. W and Ta each has a melting point of 3000.degree. C. or more, and this melting point is sufficiently higher than 1668.degree. C. and 1540.degree. C. which are the melting points of pure Ti and TiSi.sub.2 respectively. The relationship between the melting point of the metal of high melting point to be mixed and the heat resistance of silicide obtained has not been clarified at present, however, it is estimated that high melting-point metals of W, Ta, etc. having a higher melting point than Ti and TiSi.sub.2 suppress diffusion of Ti or Si at the grain boundaries to exhibit a heat resistance effect in the process that TiSi.sub.2 is constricted at grain boundaries and agglomerates as described by S. L. Hsia et al. in the paper "Resistance and Structural Stabilities of Epitaxial CoSi.sub.2 Films on (001) Si Substrates" (Journal of Applied Physics, Vol. 72, P1864-1873 (1992)).
However, as a conventional method of forming high heat-resistant silicide as described above, a target of high melting-point alloy is disposed in a sputtering apparatus in a Ti film-forming process in the salicide process as described above, an alloy film containing Ti is formed using the alloy target in the sputtering process, and high heat-resistant silicide is formed by using the salicide process. Therefore, in the case where the desired heat resistance of individual silicide is varied in accordance with the purpose of a device to be manufactured, it is necessary to prepare plural different alloy targets and exchange an alloy target to another in conformity with the device to be manufactured. Therefore, the management of the alloy target is cumbersome, and a process of exchanging the alloy target to another is required, so that the productivity of the device is lowered.